The present invention relates to thermoplastic compositions based on phenylene ether resins. More particularly, the present invention relates to thermoplastic compositions which include a blend or alloy of a phenylene ether resin, a rubber modified, high impact vinyl aromatic resin and a styrenic homopolymer.
Polymer compositions which include polyphenylene ether, high impact polystyrene and homopolystyrene are known in the art. For example, U.S. Pat. No. 3,960,808 to Katchman discloses a composition wherein the homopolystyrene has a number average molecular weight of 30,000 to 60,000. These compositions are disclosed as having an improved surface appearance over other compositions known in the art.
Similarly, compositions of polyphenylene ether resin, high impact polystyrene and homopolystyrene are shown by U.S. Pat. No. 3,819,761 to Lee, Jr. For example, Example 4 of Lee, (Col. 11), discloses a blend of polyphenylene ether, rubber modified polystyrene, and a crystal polystyrene having a number average molecular weight of 129,000.
Phenylene ether resin based compositions are well known in the art and have many uses, such as plumbing fixtures, appliance and business machine housings, as well as various automotive applications. Phenylene ether resins typically have relatively high thermal stabilities, but may suffer from deficiencies in other properties, such as poor resistance to organic solvents, poor melt flow and poor impact resistance. For this reason phenylene ether resins are often blended or alloyed with other resins or materials to give the phenylene ether resin based composition the desired spectrum of properties.
Phenylene ether resins, and phenylene ether copolymers in particular, often have high melt viscosities which limit the size of parts which may be produced and restrict the number of parts which may be molded in a given period of time, sometimes to the point where use of such resins is uneconomical. Consequently, phenylene ether resins are often combined with additional additives, such as triphenyl phosphate, mineral oil, silicon oil and polyolefins, to improve their melt flow characteristics.
As is well known in the art, enhancement of one or more properties of a thermoplastic composition is usually made at the expense of other properties, so that a composition which performs extremely well with respect to one parameter often is inadequate where other parameters are concerned. While such a skewing of performance characteristics may be acceptable for compositions intended for a few highly specialized applications, for most uses it is important that compositions exhibit a balance of various properties, rather than excelling with respect to one at the expense of others.
For example, lowering of the melt viscosity in phenylene ether resins, such as by the additives indicated above, is frequently at the expense of other properties, such as impact strength, tensile strength, and heat resistance. Consequently, a phenylene ether resin based thermoplastic composition which has good impact and tensile strength while exhibiting a lower melt viscosity offers significant practical advantages over other phenylene ether resin based thermoplastic compositions known in the art.